266 



NERVOUS SYSTEM OF VERTEBRATES. 



are very fine until they approach the point of crossing, when they 

 thicken into relatively coarse fibers. After crossing they bend 

 ventro-caudally and spread widely through the wall of the mid- 

 brain, mingling with the fiber tracts descending to the medulla 

 oblongata. In mammals the commissure arises from a nucleus 

 situated at the cephalic border of the anterior corpora quadri- 

 gemina, between the tractus habenulo-peduncularis, the nucleus 

 ruber and the root of the III nerve (Koelliker, p. 445). In other 

 vertebrates the origin of the commissure has not been clearly 

 distinguished. It is variously described as arising in, ending in or 

 serving as a true commissure for a nucleus lying cephalad from the 

 nucleus of origin of the III nerve. The same nucleus is described 



Fig. 132. Nucleus of posterior commissure in Lampetra. vent., the aqueduct of 

 Sylvius below, the optic ventricle above. 



as the nucleus of origin of the fasciculus longitudinalis medialis, 

 and the fibers of the posterior commissure itself are said to form 

 that fasciculus. These vague and conflicting statements are due 

 to imperfect methods. Those who have worked by the Weigert 

 technique have been misled by the fact that the fibers of the com- 

 missure do not become myelinated until they approach the point 

 of crossing. In Scyllium the fibers approach the median plane 

 from the lateral direction just as in Petromyzon and presumably 

 arise from a nucleus situated dorsally as in Petromyzon. The 

 same is true in amphibia. The nucleus praetectalis of Edinger 



